This invention relates to a system for detecting the threshold and spectra of substances at the atomic or molecular level.
With the advent of instruments such as the scanning tunneling microscope (STM), it is now possible to investigate the structure, spectra and dynamics of biological molecules and membranes as well as other substances at the atomic or molecular level. While more than a thousand STM's have been in operation and the instrument has sparked great interest in spectroscopy, the actual headway that has been made in this area remains rather modest. Thus, Bob Wilson and co-workers at IBM Almaden have made some progress in distinguishing closely related adsorbed surface species in STM images. G. Meijer et al., Nature 348, 621 (1990). In "Non-Linear Alternating-Current Tunneling Microscopy," Kochanski, Physical Review Letters, 62:19, pp. 2285-2288 (May 1989), a method for scanning tunneling microscopy is described, where a non-linear alternating current (AC) technique is used that allows stable control of a microscope tip above insulating surfaces where direct current (DC) tunneling is not possible.
The STM has a counter electrode on which the sample to be investigated is placed and another electrode in the shape of a microscope probe with a tip placed at a small distance away from the sample surface. A DC or a low frequency AC signal is then applied across the pair of electrodes. The probe tip is then moved across the sample surface in a scanning operation and the changes in the current or voltage across the electrodes are monitored to detect the characteristics of the sample.
The distance between the probe tip and the counter electrode/sample is controlled by a piezoelectric driver in one of two possible modes: a constaet current mode and a constant height mode. The current or voltage detected between the pair of electrodes is used to derive a control signal for controlling the piezoelectric driver in the constant current mode to change the distance between the probe tip and the sample so as to maintain a constant current between the electrodes. The voltage that has been applied to the piezoelectric driver in order to keep the tunneling current constant indicates the height of the tip z(x,y) as a function of the position (x,y) of the probe tip over the sample surface. A record of such voltages therefore indicates the topographical image of the sample surface. The constant current mode can be used for surfaces which are not necessarily flat on an atomic scale. A disadvantage of the constant current mode is the time required for the feedback loop for controlling the piezoelectric driver; this feedback action sets relatively low limits for the scan speed.
To increase the scan speed considerably, the feedback loop response is slowed or turned off completely so that the probe tip is rapidly scanned at a constant distance to the counter electrode irrespective of the contours of the sample surface. The rapid variations in the tunneling current are recorded as a function of location (x,y) to yield the topographic information of the sample surface. This is known as the constant height mode referring to the fact that the probe tip is maintained at a constant distance from the counter electrode.
The constant height mode is advantageous over the constant current mode since it has a faster scan rate not limited by the response time of the feedback loop. Consequently, slow dynamic processes on surfaces can be studied. On the other hand, it is more difficult to extract the topographic height information from the variations of the tunneling current. Furthermore, unless the sample is atomically flat, the tip might crash into a surface protrusion of the sample. For a more complete description of the two operating modes of the STM's, please see "Scanning Tunneling Microscopy I," by H. J. Guntherodt R. Wiesendanger (Eds.) , Springer-Verlag, pp. 5-6.
In the article referenced above, Kochanski proposes to investigate insulating films by applying an AC current between the electrodes at frequency .omega. and the current between the electrode at 3 .omega. is detected. The AC signal is generated using a 2 GHz resonant cavity so that the frequency or frequencies of the signal applied to the STM electrodes and detected must be fixed in the scanning operation performed by Kochanski.